The buff

Hello, I am French and I have certainly not an English as good as yours. but let

Here I have coding variants at the entrance of icc. But when I activate the 10% buff it appears everything is good, and when I activate the buff 15%, the 10% buff does not disappear.

What is the solution please?

#include "ScriptMgr.h"
#include "ScriptedCreature.h"
#include "ScriptedGossip.h"
#include "Player.h"

enum Spells
{
	//Buff alliance
	spell_force_5 = 73762,
	spell_force10 = 73824,
	spell_force15 = 73825,
	spell_force20 = 73826,
	spell_force25 = 73827,
	spell_force30 = 73828,

	//Buff horde
	spell_chant_5  = 73816,
	spell_chant_10 = 73818,
	spell_chant_15 = 73819,
	spell_chant_20 = 73820,
	spell_chant_25 = 73821,
	spell_chant_30 = 73822,
};

class npc_varian : public CreatureScript
{
public:
	npc_varian(): CreatureScript("npc_varian"){ }
	
	struct npc_varianAI: public ScriptedAI
	{
		npc_varianAI(Creature* creature) : ScriptedAI(creature) { }



		void sGossipSelect(Player* player, uint32 /*sender*/, uint32 action) OVERRIDE
		{
			if (action == 2)
			{
				player->CLOSE_GOSSIP_MENU();
				player->InterruptNonMeleeSpells(false);
				player->CastSpell(player, spell_force_5);
	
			}

			if (action == 3)
			{
				player->CLOSE_GOSSIP_MENU();
				player->InterruptNonMeleeSpells(false);
				player->CastSpell(player, spell_force10);
			}

			if (action == 4)
			{
				player->CLOSE_GOSSIP_MENU();
				player->InterruptNonMeleeSpells(false);
				player->CastSpell(player, spell_force15);
			}

			if (action == 5)
			{
				player->CLOSE_GOSSIP_MENU();
				player->InterruptNonMeleeSpells(false);
				player->CastSpell(player, spell_force20);
			}

			if (action == 6)
			{
				player->CLOSE_GOSSIP_MENU();
				player->InterruptNonMeleeSpells(false);
				player->CastSpell(player, spell_force25);
			}

			if (action == 7)
			{
				player->CLOSE_GOSSIP_MENU();
				player->InterruptNonMeleeSpells(false);
				player->CastSpell(player, spell_force30);
			}
		}
	};
	CreatureAI* GetAI(Creature* creature) const OVERRIDE
	{
		return new npc_varianAI(creature);
	}
};
void AddSC_buff_icc()
{
	new npc_varian();
}